Facebook addicts who frequently check the app on their smartphones tend to have less gray matter in the "reward centers" of their brains, according to a study published in medical journal Behavioral Brain Research.

The nucleus accumbens, located in one's basal forebrain, is part of the organ's mesolimbic pathway, which is activated during pleasurable experiences. Each cerebral hemisphere has its own nucleus accumbens, which is comprised of a shell and a core.

During a five-week study, researchers from Germany's Ulm University and the University of Bonn monitored 46 male and 39 female Facebook users, tracking the length of time each subject spent on the popular social media site and number of times they "checked in."